1. Field of the Invention
This invention relates to non-destructive evaluation of electrically conductive objects by detecting the effects of flaws on the magnetic field generated by a current passed through the object. More particularly, the invention is directed to such a method and apparatus which includes cancellation of the magnetic fields produced by the edges of objects and the fields surrounding cylindrical objects to improve the relative signal strength of magnetic field disturbances attributable to flaws.
2. Background Information
One technique for non-destructive evaluation of electrically conductive objects is to pass an electric current through the object and observe disturbances in the magnetic field produced by the current resulting from flaws. Typically the component of the field perpendicular to the surface of the object is measured by a magnetometer. Superconducting quantum interference device (SQUID) magnetometers are very sensitive instruments which have been applied to detecting the very small disturbances in the magnetic field produced by flaws. However, features of the conducting object can generate much larger normal field components than the flaws. For instance, the edges of a conductive plate generate a large magnetic field component perpendicular to the surface of the plate. Also, the fields generated around rods and tubes and other such objects have a large normal component. These normal field components produced by features of an unflawed object mask the field generated by a flaw and thus reduce the sensitivity of such detection techniques.
The magnetic field surrounding the electric leads which apply the current to the conducting object is even greater than the field produced by the edges or the field surrounding a cylindrical object, because of the higher current density in the leads. As a result, it is even more difficult to locate flaws in the vicinity of the leads supplying current for the testing for flaws.
Thus, despite the high sensitivity of SQUID magnetometers, there have been limitations on the minimum size of flaws in the conducting object that can be detected by monitoring the magnetic field generated by a current applied to the object.
There is a need therefore for an improved method and apparatus for the non-destructive testing of conducting objects.
There is a more particular need for such method and apparatus which substantially reduces the effects of edges and of closed surfaces of an object on the magnetic field produced by a current passed through the object.
There is a further need for such a method and apparatus which can be used to test for flaws in elongated conducting objects as they are fed through a test station.